Osmosis

Osmosis - The movement of water molecules from a reigon of high water potential to a reigon of low water potential (Down a water potential gradient), through a parcially permiable membrane.

Because of this water potential gradient, plants are able to draw water up from the soil this is because the root hair cells use Active Transport (using ATP) to uptake minerals, which reduces the water potential gradient and eventually is lower than the soil. Water is then diffused across the parcially permiable membrane and through the roots.